1. Field of the Invention
The present invention relates generally to transmission systems and more specifically to a technique for improving the carrier-to-noise ratio (CNR) of radio transmissions. The present invention is particularly suitable for improving the CNR value of a code division multiple access (CDMA) transmitters such as cell-site base transmitters.
2. Description of the Related Art
In cell-site CDMA transmitters of a cellular mobile communication network, individual spread spectrum channel signals are synchronised and multiplexed (or digitally summed) into digital amplitude data. This amplitude data is converted by a digital-to-analog converter to an analog amplitude signal, which is up-converted, power-amplified and then transmitted. In order to prevent the digital-to-analog converter from being overloaded, the maximum amplitude of each transmit channel signal is determined in advance so that the total amplitude of the multiplexed signals corresponds to the upper limit of the digital-to-analog converter. However, the performance of the transmission system is determined by the CNR requirement of the largest signal level to be power amplified prior to modulation onto a radio frequency carrier. As a result, when not all individual channels are multiplexed during low traffic periods, the carrier-to-noise ratio of the system becomes lower than the required value.
One approach to this problem is to use a digital-to-analog converter for each channel signal to produce a multiplex of analog amplitude signals, instead of using the single digital-to-analog converter for producing a multiplex of digital amplitude signals. However, this approach is not an economical implementation and requires additional circuitry that ensures orthogonality between any pair of analog spread spectrum channel signals.